Most of the present mobile communication systems offer data and speech services based upon circuit switched technology. In the circuit switched technology the communication connection is maintained during the whole connection although no information is transferred at times. This unnecessarily reserves transfer resources also shared by many other users whereby maintaining a circuit switched connection to one subscriber unnecessarily reserves transfer resources of the other users. Another disadvantage in the circuit switched technology is the burstlike characteristic of the data services. Packet switched information transfer is known for making channel utilization more effective.
As well as the fixed networks shall, the future mobile communication network shall be able to relay both circuit switched and packet switched transfer, e.g. ISDN-transfer (Integrated Services Digital Network) and ATM transfer (Asynchronous Transfer Mode). A protocol based on PRMA (Packet Reservation Multiple Access) for relaying packet switched information is known in mobile communication networks. It is also called "Packet Radio". The PRMA is a technology for multiplexing packet formatted digital speech or data into a time divided carrier wave. In other words, PRMA uses in a radio channel Time Division Multiple Access (TDMA) in which transmission and reception take place time divided at predetermined times. The PRMA protocol has been developed for utilizing the discontinuity of voice transfer to support more speech users than the speech channels of a time divided carrier wave provide for. In this case a channel is allocated to a mobile station, e.g. a speech channel when speech is produced and when speech ends the channel is released so that a mobile station does not unnecessarily reserve capacity but the channel is free for use for other purposes, e.g. for transmissions of other mobile stations in the cell. The PRMA protocol is used in cellular mobile communication systems for communication between a mobile station and a base station. The communication link from a mobile station to a base station is referred to as uplink and the communication link from a base station to a mobile station is referred to as downlink.
The known mobile communication systems based upon Time Division Multiple Access (TDMA) utilize usually either TDD (Time Division Duplex) or FDD (Frequency Division Duplex). In a system utilizing the TDD both uplink and downlink transfers are effected usually at the same frequency with time division, that is at different times. The transfer of information is effected in the time slot of a so called TDMA frame, meaning that a mobile station is transmitting in the uplink in e.g. one time slot and the base station is transmitting in the downlink in another time slot. In several communication systems based upon circuit switching e.g. time division is realized by allocating a certain time slot of a frame to a certain mobile station for both uplink and downlink communication and by delaying the uplink frame e.g. by a few time slots compared with the downlink frame, resulting in the same time slots in the uplink frame and the downlink frame not overlapping each other. In this kind of system uplink and downlink communication are dependent on each other, meaning that time slots for a mobile station are allocated in pairs for uplink and downlink, in other words always simultaneously both the transmission and the reception slot. Thus it can be ensured that no collisions occur between uplink and downlink transmissions. On the other hand this kind of procedure is inefficient utilization of resources. If, in addition to this, allocating or reserving of time slots for a certain mobile station is symmetric, it means that a mobile station transmits and receives in equally many time slots in the uplink and in the downlink.
On the other hand, systems are known, such as the system based upon the PRMA protocol and the GSM GPRS (General Packet Radio Service) which is described in more detail later, in which the uplink and downlink transmissions are independent of each other, meaning that a mobile station is allocated separately a transmission time slot for uplink communication and separately a reception time slot for downlink communication, completely independent of each other.
In a system utilizing the above mentioned Frequency Division Duplex (FDD) the uplink and downlink transfers are effected at different frequencies. In a system of this kind transmission and reception can take place simultaneously. On the other hand, there are systems, such as GSM, which utilize both time division and frequency division, meaning that even if transmission and reception take place at different frequencies they do not occur simultaneously.
A problem in the systems, the uplink and downlink transmissions of which are independent of each other, are collisions, meaning that the uplink and downlink transmissions overlap. In systems utilizing independent uplink and downlink transmissions the base station does not know which mobile station is transmitting in the uplink simultaneously when the base station should transmit a packet in the downlink. This kind of a situation is presented in FIG. 1 in which downlink and uplink transmissions are independent of each other and in the figure in the center TDMA frame the uplink and downlink have simultaneous communication to and from the same mobile station, resulting in a collision and at least one of the transmissions is lost. For example, in the above described packet radio system which utilizes uplink and downlink transmissions independent of each other, the packets are formed at moments almost impossible to predict. Likewise, the packets sent by the base station may arrive to the mobile station completely unpredictably. In this case the uplink and downlink transmissions can collide resulting in the loss of data to at least one direction. Collisions may occur between different data flows or between the packets of the same connection, if e.g. a mobile station acknowledges the previous transmission, whereby the acknowledgement in the uplink and the next transmission in the downlink may collide. There is a method for avoiding this kind of collisions, in which a mobile station after each of its transmissions waits for the acknowledgement from the base station before its next transmission. However, it is possible that during the next uplink transmission of the mobile station there is a simultaneous transmission from the base station to the mobile station in the downlink. Another alternative to avoid collisions would be to equip a mobile station with two transceivers. However, this is not desirable because of cost and size. A base station is normally equipped with several transceivers ant that is not a problem, but the real problem lies with mobile stations equipped with one transceiver, which, because of the solution used, cannot send and receive completely simultaneously and are thus incapable of a full-duplex connection.